As described by Sullivan & Lynch in U.S. Pat. No. 5,199,424, issued on Apr. 6, 1993, the application of continuous positive airway pressure (CPAP) has been used as a means of treating the occurrence of obstructive sleep apnea. The patient is connected to a positive pressure air supply by means of a nose mask or nasal prongs. The air supply breathed by the patient is provided at a pressure that is slightly greater than atmospheric pressure. It has been found that the application of continuous positive airway pressure provides what can be described as a “pneumatic splint”, supporting and stabilizing the upper airway and thus eliminating the occurrence of upper airway occlusions. Such a treatment pressure can be effective in eliminating both snoring and obstructive sleep apnea and in many cases, is effective in treating central and mixed apnea.
Detecting apneas and more particularly the type of apnea that a patient is experiencing may be important, as certain types of apneas need to be treated differently.
As described by Berthon-Jones in U.S. Pat. No. 6,675,797, issued on 13 Jan. 2004, central and obstructive apneas can be detected (and distinguished) using a forced oscillation technique. Airway patency is determined by applying an oscillatory pressure waveform and then determining if a component of the air flow indicates that a central or obstructive apnea is occurring. For example, the component may be the amplitude of the air flow signal, and when comparing the patient's amplitude, or its derivative, to a threshold value, this will indicate if a central or obstructive apnea is occurring.
As described by Berthon-Jones in U.S. Pat. No. 7,730,886, issued on 8 Jun. 2010, central apneas can be distinguished from obstructive apneas by analyzing the air flow signal and identifying if there is a cardiogenic component of the air flow signal. For example, if no cardiogenic component of the air flow signal can be detected, the patient will received pressurized breathable gas to treat an obstructive apnea.
Despite the availability of such methods for detecting and distinguishing obstructive and central apnea, some sleep disorder breathing events may still go undetected and thus untreated with the use of some devices. Thus, it will be appreciated that there may be a need for improved techniques and devices for addressing the conditions of sleep disordered breathing.